Calutron with means to direct calcium gettering vapor into the ion beam to reduce tank pressure



A nl 2, 1968 w. A. BELL, JR., ETAL 3,376,414

CALUTRON WITH MEANS TO DIRECT CALCIUM GETTERING VAPOR INTO THE ION BEAM TO REDUCE TANK PRESSURE Filed June 4, 1965 2 Sheets-Sheet 1 ION RECEIVER CALCIUM VAPOR CLOUD BAFFLE ASSEMBLY TO VACUUM -a MAGNETIC FIELD OUT FROM PAPER W 5 CALCIUM BOILER INVENTORS. William A. Bell, Jr. BY King A. Spainhour Allen M. Veach KM a 0 ATTORNEY.

VAPOR April 2, 1968 w. A. BELL, JR. ET AL MEANS TO DIRECT CALCIUM GETTERIN G ESSURE CALUTRON WITH INTO THE ION BEAM TO REDUCE TANK PR 2 Sheets-Sheet 2 Filed June 4, 1965 r s R :0 oH T m M mam 5 mA 0 4 mhm WK A ATTORNEY.

3,376,414 CALUTRON WITH MEANS T DIRECT CALCIUM GETTERING VAPOR INTQ THE ION BEAM TO REDUCE TANK PRESSURE William A. Bell, 31"., King A. Spainhour, and Allen M. Veach, Oak Ridge, Terran, assignors to the United States of America as represented by the United States Atomic Energy Commission Filed June 4, 1965, Ser. No. 461,568 4 Claims. (Cl. 25041.9)

ABSTRACT 0F THE DISCLOSURE A calcium boiler is positioned within the tank unit of a calutron such that the calcium vapor from the boiler is directed toward the calutron ion beam and the vapor encompasses the greater portion of the ion beam. The gettering action of the calcium helps to further reduce the calutron tank pressure. However, the ion outputs for most isotopes, which are separated during use of the calcium boiler, are increased by at least tenfold over what was expected on the basis of improved vacuum alone, and ion beam resolutions are also greatly improved.

It has been shown that isotopes of many chemical elements can be separated and the desired elements enriched in the electromagnetic mass spectrometer known as the calutron, described in the E. 0. Lawrence patent, No. 2,709,222. In'the calutron, described in that patent, there is provided an ion-producing means wherein a feed or charge material is converted into ions for subsequent acceleration through a magnetic field and separation into the respective masses of the isotopes present in the charge material. Since most charge materials are solids, the charge must first be heated in a vaporizer or oven to produce a charge vapor. Vapor from the vaporizer or oven passes to an arc chamber where a stream of electrons (commonly called the arc discharge) is passed through these vapors, ionizing them for subsequent acceleration through a magnetic field. One means for heating the charge material for use in the ion source is dis closed in US. Patent No. 3,115,575 to William A. Bell, et al., issued Dec. 24, 1963.

The contamination of separated isotopes with other isotopes and apparently unseparated material has been a perpetual problem in calutron operations. Many ideas.

have been put forth as to the cause of this contamination. Included is the idea of contamination by unseparated material that may be drawn along with the ion beam. Another is that related to charge exchange caused by high pressure along the path of the ion beam.

In some isotope separations it is known that the feed material has a high vapor pressure and in these instances this is probably a contributing factor in contamination. Also, the resultant higher-than-desired tank pressure necessitates smaller-than-desired outputs to achieve the best isotopic separation. Accordingly, it is well recognized that a reduction of tank pressure is highly desirable for separations using these more volatile compounds. Specific separations for which this problem exists are the separation of the isotopes of titanium, tin, mercury, and sulfur, for example.

The most satisfactory prior art method of reducing the problem for mercury separations is that of using a very cold tank liner as cooled with refrigerated alcohol. This, of course, is very expensive. Another general solution to the problem that has been tried has been through the use of additional strategically located dilTusion-type vacuum pumps. This has proven to be of little value due to the capacity of such pumps. Thus, the reduced output has States Patent 0 3,376,414 Patented Apr. 2, 1968 ice been relied upon generally for achieving the best assay of the separated material. This, of course, greatly increases the length of time required to obtain a given quantity of separated isotopes.

With a knowledge of the limitations of prior calutron systeins regarding the problem of providing adequate tank pressure for efiicient separations using feed materials having a high vapor pressure, it is a primary object of the present invention to provide a means for improving the vacuum pumping for calutrons.

It is another object of the present invention to provide a means for not only improving the vacuum pumping for calutrons, but also greatly improving the efliciency of separations in the operation of such oalutrons, such as increasing the ion output and the ion beam resolution.

These and other objects and advantages of the present invention will become apparent upon a consideration of the following detailed specification and the accompanying drawing, wherein:

FIG. 1 is a schematic diagram of a calutron unit provided with a calcium boiler, and

FIG. 2 is a cross-sectional view of the calcium boiler of FIG. 1.

The above objects have been accomplished in the present invention by providing a calcium boiler within the tank unit of a calutron positioned in such a manner that the calcium vapor from the boiler is directed toward the ion beam from the ion source unit of the calutron such that the vapor sees the greatest portion of the ion beam' from the ion source. The gettering action of the calcium helps to further reduce .the tank pressure of the calutron. However, the expected improved operation on the basis of improved vacuum is a l0-20% increase in the ion output for the isotopes of sulfur, for example, while actually an improvement of about 300% in ion output was attained when using the calcium boiler in a calutron. This indicates that factors other than pumping are involved. In fact, for some separations using the calcium boiler, there are improvements in calutron operation that occur even though little change in base pressure is effected, which will be discussed more fully below. Thus, it should be evident that the use of a calcium boiler in a calutron provides for not only better tank pressure therein, but also provides for a substantially more efiicient operation ofa calutron that is substantially better than that expected on the basis of improved vacuum alone.

Referring now to FIG. 1 of the drawings, a conventional. calutron has an ion source 1 and a receiver 2 mounted from faceplates, not shown, and inserted Within a vacuum tank 3. The tan-k 3 is evacuated by means of a vacuum pump, not shown, in a conventional manner. It should be understood that the ion receiver 2 is provided with several collection pockets, not shown, in a conventional manner to receive the separated isotopes as separated by the magnetic field encompassing the tank 3 with the field being provided by means, not shown, in a conventional-manner. For example, the magnetic field may be provided for the unit of FIG. 1 in the same mannet as set forth in the above-mentioned Lawrence patent. A typical ion beam 4 is shown passing through a bafiie assembly 5 positioned midway between the ion source 1 and the receiver 2. A support arm 6, extending from the source'l, carries the calcium boiler 7. This boiler is positioned near the ion beam so that the wedge-shaped vapor cloud emanating therefrom seesthe greatest por- The boil-er 7, as shown in detail in FIG. 2, is fabricated in a manner similar to a conventional ion source in that a charge bottle 8 is heated by a first set of heaters 10, and the vapor exit area 22 of the boiler is maintained at a. higher temperature by a second set of separate heaters 18, 18.. No ionization or acceleration is provided. The exit slit 17 of the vapor area 22 is so shaped that the vapor is exited with a cross section having about a 30 included angle. The boiler has a capacity of vaporizing 3-5 grams of calcium per hour, and is normally operated with about 650 watts of power to the first set of heaters and 1100 watts to the second set of heaters. All portions of the boiler which are in contact with the calcium metal or vapor that is, the member 14, the top plate the vapor exit nipple 21 connecting the charge bottle 8 to the area 22, the housing 9 for the charge bottle, and the member which threadedly engages the nipple 21, are fabricated from, or protected by, stainless steel. The remainder of the boiler, including the housing 12, the member 23, the lower heater insulators 11, 11 and the upper heater insulators 19, 19', is graphite. Heat shielding 13 is provided in the lower part of the boiler encompassing the charge bottle 8, and heat shieldings 16, 16' are provided about the heaters 18, 18'.

In a normal utilization of the calcium-getter pump, the initial operational procedures which are conventional in the calutron art are followed, including insertion of the ionsource 1' and receiver 2 in the tank 3, and thereafter evacuating the tank to as low a base pressure as is practical. A support gas arc is struck in the ionization region of the ion source 1, followed by the introduction of a small amount of feed material to the source to create a weak ion beam. The are and ion beam accelerate the outgassing of the calutron components. After the outgassing is completed, the feed material to the ion source 1 is increased to a normal operating value and the calcium boiler 7 is then gradually brought to a proper operating temperature. This operating point is a temperature just below that at which calcium vapor droplets are big enough to spark" as they intercept the ion beam. This operating temperature may have to be adjusted from time totime as the ion beam varies throughout the duration of a separation run.

The results obtained using the calcium getter pump are better than expected when utilized during the enrichment of S. The feed material for this separation was CS When the calcium pump was activated the tank pressure dropped an amount equivalent to the pumping capacity of at least eight 20-inch oil diffusion pumps. At the same time, the quantity Q in milliamperes of the S ion beam increased by a factor greater than three. The expected or hoped-for improvement of 10-20% in the ion beam output as a result of'the use of the calcium pump was thus surpassed by the greater-than-l00% improvement obtained. The following table shows the eifect upon pressure and the quantity Q, in a typicalrun, Q being the total ion current for all the isotopes collected.

From theabove table, it can be seen that the vacuum was improved by a factor of 400%, While at the same time the amount of sulfur ions monitored in the collection pockets increased more than 300% when the Ca pump was utilized. It should also be noted that the previous maximum purity of S of less than of this collected isotope without the Ca pump was bettered by the routine assay of about 70% with some assays approaching 80% when the Ca pump was utilized. When the charge material was SiS the average sulfur ion current was increased from 20 ma. to ma., and the S assay was increased from a previous high of 44% to 89% as a result of operating the Ca pump.

For silicon isotope separations, the preferred charge material for these separations is SiS and, when the calcium vapor generator is utilized, the average ion current for the Si isotopes is routinely doubledfrom 6O ma. to ma. The average assay for Si was about 94.0% with some assays as high as at least 97%, and the average assay for Si was about 95.0% with some assays as high as at least 97.5% when using the calcium boiler in the calcutron, and these assays represent about a twofold increase in enhancement over that achieved without the calcium boiler.

For tungsten isotope separations, the charge material is a tungsten chloride which may be WCl' or WCI for example. When the calcium vapor generator was used, the vacuum was increased from 1X10 torr to 2'.5 10 torr, and the ion current at the receiver was approximately doubled over that achieved without the vapor generator. In addition, the enrichment index for W'was increased by a factor ofabout 3, with the assay increased from 94% to 97.6%.

For titanium isotope separations, the charge material is TiCl and, when the calcium. vapor generator is used, the pressure is substantially reduced and the assays for Ti and Ti were 90.1% and 87.8%, respectively, as compared with 86.4% and 84.1%, respectively, that could be achieved with the calutron operating without the calcium vapor generator.

For mercury isotope separations, the use of a calcium vapor generator has shownsignificant effect upon these separations without much apparent effect upon the vacuum-onl'y a few divisions on the pressure gauge dial.

Assays of the collected material also indicate little pumping of the background gas. A significant effect upon the resolution at the calutron receiver has been observed, however. Using the Hg ion beam as the observed quality ofseparation, an average of 6.0 ma. was maintained during. a 40-hour separation run. The total current for all beams averaged 20; ma. During this time, the valley between Hg and Hg was 0.15 ma., giving a peakto-valley ratio of 40. In no previous run with a prior art calutron has the I-Ig ion beam been resolved when the total current is in excess of 6 ma. or resolved as well as above whenthe total current was greater than 3 ma.

In previous runs in the XAX calutrons (double the separation distance) without the calcium generator, 98.3% Hg material was collected when alcohol-refrig erated liners were used, and the assay fell to 69% when the. liners were not cooled. In the. 255 calutron with the liner cooled, the I-Ig assay was 83%. On the first run using the calcium vapor generator, with no liner refrigeration, the assay was 86.7% at 10 times the collection rate. Thus, it can be seen that the assay of collected isotopes of mercury is significantly increased when using the calcium vapor generator without liner cooling, and, if the desired assay can be achieved or approached without the alcohol cooling, at saving of the order of hundreds of dollarsv per week can be realized during the separation of desired mercury isotopes including Hg. On the other hand, the increased output using the calcium vapor generator almost compensates for the cost of cooling if such is required.

The charge material used for the above mercury. separations was HgS. However, HgCl charge material can also be and has been used to provide operating results that are closely comparable to those obtained with HgS.

The use of the HgS chargematerial is preferred since it provides for slightly better, resolved ion beamsthan that obtained with the HgCl material when using the calc1um vapor generator.

The improvements that are obtained when a calcium vapor generator is utilized, in a calutron, as discussed above, may be summarized as substantially increased ion output, improved ion beam resolution, and/or lowered background pressure, and, as pointed out above, there are improvements in calutron operation for mercury separations that occur even though little change in base pressure is effected. In those separations where a substantial change in base pressure is effected when the calcium generator is used, the expected ion beam outputs based upon pressure reduction alone have been substantially exceeded, thus providing a unique and very efficient method for the separation of isotopes with the assays of the collected isotopes being substantially greater than those obtainable with prior art calutrons.

It should be understood that the present invention is not limited for use with just the highly volatile compounds as discussed above. For example, the calcium pump will pump most, if not all, of the halide ions, particularly chlorine. Thus, S Cl can be and has been used as a feed material for the collection of sulfur isotopes. Thus, since about 75% of all separations utilize a chloride feed material, the calcium pump can be used for most separations involving not only the highly volatile compounds, but also those compounds that have a low vapor pressure.

Since the calcium pump not only reduces the background contamination, but also reduces the background pressure in most separations, it permits the use of more optimum are conditions which provides for even more eificient operation. An additional advantage that accrues from the present invention is the retention of material on the tank walls, due to the formation of compounds with the getter vapor, rather than being removed into the conventional vacuum system. When recycle of material is necessary, this is a very important feature. The deposited vapor may also contribute secondary electrons that assist in space charge neutralization in the ion beam.

This invention has been described by way of illustration rather than by limitation and it should be apparent that the invention is equally applicable in fields other than those described.

We claim:

1. An improved system for substantially increasing the ion output, providing substantially better ion beam resolution, eliminating any possible need for calutron liner cooling, and for lowering the background pressure in a calutron including a vacuum tank; an ion source and an ion receiver disposed within said tank, said ion source being adapted to receive a charge material; evacuating means connected to said tank; means for providing a magnetic field encompassing said tank for effecting a charge separation of the ion beam from said source; the improvement comprising a calcium boiler containing calcium and being mounted adjacent to said ion source, and heater means associated with said boiler for continuously vaporizing said calcium, a selected amount of said calcium being vaporized per hour, said boiler being provided with a vapor exit region for directing a calcium vapor into the path of said ion beam and encompassing the greatest portion of said ion beam, whereby said calcium vapor effects a reduction in the background pressure, provides for better resolution of said ion beam at said ion receiver, and effects an increase in the ion output of said calutron up to at least 300%.

2. The system set forth in claim 1, wherein said calcium boiler is provided with a charge bottle containing said calcium, a first set of heaters for heating said bottle, and a second set of heaters for heating said vapor exit region to a higher temperature than said bottle, said exit region being so shaped that the vapor exited therefrom has a cross section having about a 30 included angle.

3. The system set forth in claim 2, wherein said boiler has a capacity of vaporizing 35 grams of calcium per hour, said first set of heaters being operated with about 650 watts of power, and said second set of heaters being operated with about 1100 watts of power.

4. The system set forth in claim 2, wherein said charge material to said ion source is selected from the group consisting of CS S CI SiS TiC1 W01 WCl HgS, and HgCl References Cited UNITED STATES PATENTS 1,638,551 8/1927 Ronci 313180 2,901,618 8/1959 Ludwig et al. 25041.9 2,921,199 1/1960 Davidson 25041.9

RALPH G. NILSON, Primary Examiner. 

